1. Field of the Invention
The present invention relates to a novel dry positively chargeable toner for developing electrostatic latent images for use in electrophotography, electrostatic recording, electrostatic printing, etc.
2. Description of the Prior Art
Electrostatic latent images can be developed into visible images by depositing a toner on the latent image by electrostatic attraction. Powder developers as well as liquid developers are widely used for developing electrostatic latent images. Powder developers can be divided generally into two-component developers and single-component developers. The two-component developer comprises a particulate toner having a mean particle size of 15 .mu.m and prepared by dispersing a coloring agent, charge control agent, fluidizing agent and the like in a natural or synthetic resin, and a carrier of finely divided iron, ferrite or the like admixed with the toner and having a particle size of 100 to 200 .mu.m. The latter single-component developer comprises only a particulate toner having a mean particle size of 15 .mu.m and prepared by dispersing a coloring agent, charge control agent, fluidizing agent and the like in a natural or synthetic resin.
With the two-component developer, the toner is triboelectrically charged by the carrier and deposited on electrostatic latent images for development. The toners heretofore known and serving as single-component developers include those which are triboelectrically chargeable by a brushlike or platelike friction member having the same function as the carrier and serving as a substitute therefor. Further provided in recent years are toners which are triboelectrically chargeable by a dispersed finely divided magnetic material. These developing toners are charged positively or negatively in accordance with the polarity of the electrostatic latent image to be developed.
To enable the toner to retain the charge, it is also proposed to utilize the triboelectric chargeability of the resin used as the main component of the toner, but the toner so adapted is low in chargeability and has a great solid surface resistance value. Consequently the toner image obtained is prone to fogging and obscure. Although a toner is known in which a certain functional group is introduced into the resin to enable the toner to retain positive charges, the toner releases a disagreeable odor during thermal fixing due to the functional group and is subject to variations in the amount of charges retainable. The charge control agents presently used in the art for giving positive chargeability include the nigrosine dyes disclosed in Examined Japanese Patent Publication SHO 41-2427, etc., the quaternary ammonium salts disclosed in U.S. Pat. No. 3,565,654, etc. and polyamide resins disclosed in Examined Japanese Patent Publication SHO 53-13284.
However, many of these charge control agents heretofore used are colored and are therefore not suited to color toner compositions although useful for black toner compositions. Further the above-mentioned quaternary ammonium salts provided as colorless charge control agents are soluble in water, difficult to disperse uniformly in the toner and prone to uneven charging. Further even if the toner incorporating the agent has high chargeability immediately after preparation, the chargeability decreases exponentially functionally depending on the conditions under which the toner is stored, and this tendency becomes more pronounced when the toner is exposed to a high temperature and high humidity. To overcome these drawbacks, improvements have been made in charge control agents as disclosed, for example, in Unexamined Japanese Patent Publications SHO 56-11461, SHO 57-54953, SHO 57-119364, SHO 58-9154 and SHO 58-98742, etc. Despite these research efforts, nevertheless, a fully satisfactory charge control agent still remains to be developed.